The application of the results of x-ray diffraction investigations of the structures of biologically active substances in order to discuss and, hopefully, to explain, the specific reactivities of compounds such as proteins, viruses, antibiotics, drugs, and the like, has led to some notable successes in the past, e.g., for polypeptides by Pauling and Corey, and for DNA by Watson and Crick. In both of these structures the knowledge of hydrogen bonding, as obtained from research on the structures of simpler substances, was probably the most important factor in the success of the above-mentioned investigators. There remain, however, some aspects of this important structural feature which are incompletely understood at the present time, and which warrant further investigation. These include: C-H - O sytems, "Symmetrical" hydrogen bonds, strain in hydrogen bonds, and the requirement of complete hydrogen bonding. Equally important to hydrogen bonding in these biological structures are the various possible conformations about carbon-carbon and sulfur-sulfur single bonds. The torsion angles in disulfide linkages, an important feature in many proteins, are still not fully understood at the present time. It is proposed to analyze the very numerous published structures which contain one or more of the above features in order to add to the understanding of them. When, as the past, redetermination of a structure seems indicated, this will be done by the collection of new data and refinement thereof by the usual procedures. It is also proposed to carry out crystal structure determinations of new substances in which the features of interest are present.